Generally, transistors are considered as the basis of modern electronics. Transistors typically used for switching in modern computers are based on the field effect. In such transistors, a voltage applied between the gate and drain electrodes increases the conductivity of a semiconductor, allowing electricity to flow between the source and drain electrodes. A transistor should ideally carry as little current as possible when there is no voltage between the gate and drain (the off state) and as much as possible when gate voltage is present (the on state). A low off current is important for energy efficiency, while a large on current is important because it allows circuits to run faster.
Therefore, an important measure of the quality of a transistor is the ratio of the current when the transistor is in on state (“ON” current) to the current when the transistor is in “OFF” state (“OFF” current). However, with a standard field-effect transistor (FET), this change in conductivity is influenced by only a thin layer close to where the current flows between gate and drain. This limits the ratio of on current to off current that can be achieved.
As transistor geometries shrink, the voltage that can be applied to the gate is reduced to maintain reliability. To maintain performance, the threshold voltage of the FET is reduced as well. As threshold voltage is reduced, the transistor cannot be switched from complete turn-off to complete turn-on with the limited voltage swing available. Typically, the circuit design is a compromise between strong current when the transistor is in the “ON” state and low current when the transistor is in the “OFF” state. Subthreshold leakage (including subthreshold conduction, gate-oxide leakage and reverse-biased junction leakage) typically can consume upwards of half of the total power consumption of high-performance integrated circuit chips.
In the subthreshold operating regime of the FET, when the gate voltage is lower than a threshold, the drain current versus gate voltage behavior is typically approximated by a linear curve. The slope of this curve is referred to as a subthreshold slope.
Conventional metal-oxide-semiconductor field effect transistors (“MOSFETs”) typically have the subthreshold slope that is thermally limited to about 60 mV per decade. That is, for typical FET transistors increase of the gate voltage by about 60 mV causes increase the drain current by less than about a factor of 10. The limited subthreshold slope cannot provide fast transition between off (low current) and on (high current) states of the transistor.